On 01/12/2019 01:05, Arvind Sankar wrote: > On Sat, Nov 30, 2019 at 11:23:52PM +0300, Pavel Begunkov wrote: >> bvec_iter_advance() is quite popular, but compilers fail to do proper >> alias analysis and optimise it good enough. The assembly is checked >> for gcc 9.2, x86-64. >> >> - remove @iter->bi_size from min(...), as it's always less than @bytes. >> Modify at the beginning and forget about it. >> >> - the compiler isn't able to collapse memory dependencies and remove >> writes in the loop. Help it by explicitly using local vars. >> >> Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx> >> --- >> >> v2: simplify code (Arvind Sankar) >> > > Thanks :) > > Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx> > > Btw, I discovered that gcc 9.2 doesn't optimize away the second > comparison in something like That's unfortunate. IDK, whether it could be easily done in gcc, but maybe relay it to compiler guys? > > m = min(a,b); > return m>a; > > So the WARN_ONCE bit doesn't get optimized away even in cases like > bio_for_each_bvec where it's guaranteed at compile-time to not trigger. > That's more like a starting point. The idea is to revise and tune/rewrite iteration helpers including *for_each_bvec(). I assume, all those are really poorly optimisable (especially with -fno-strict-aliasing). So, noted to consider -- Pavel Begunkov
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